Hearing-impairment is a significant health problem, particularly at the two ends of the human life span. Approximately one in a thousand newborn infants and more than a quarter of adults over the age of 65 have a significant hearing loss. In the case of infants, early detection of hearing loss is necessary to ensure that appropriate treatment is provided at an early stage and that the infant can develop normal speech and language. The detection of a hearing-impairment requires a measurement of how well someone can hear sound (i.e. audiometry).
Conventional audiometry is performed by having a subject respond to acoustic stimuli by pressing a button, saying “yes”, or repeating words that may be presented in the stimulus. These tests are subjective in nature. Audiometry allows an audiologist to determine the auditory threshold of the subject, which is defined as the lowest intensity at which a sound can be heard. The audiologist evaluates the auditory threshold of a subject by using a stimulus that most commonly consists of a pure tone. The stimulus is presented via earphones, headphones, free field speakers or bone conduction transducers. The results are presented as an audiogram which shows auditory thresholds for tones of different frequencies. The audiogram is helpful for diagnosing the type of hearing loss a subject may have. The audiogram can also be used to fit a hearing aid and adjust the level of amplification of the hearing aid for subjects who require hearing aids.
Audiometry may also involve subjective testing at supra-threshold intensities to determine how well the subject's auditory system discriminates between different sounds (such as speech) presented at intensities at which they normally occur. The audiologist will therefore determine how many simple words a subject can accurately recognize at different intensities with and without different amounts of background noise. The audiologist may also conduct tests which measure how well the subject can discriminate changes in the intensity or frequency of a sound or how rapidly these changes occur.
Conventional audiometry cannot be performed if the subject is an infant, young child or cognitively impaired adult. In these cases, objective tests of hearing are necessary in which the subject does not have to make a conscious response. Objective audiometry is essential for detecting hearing impairment in infants or elderly patients as well as for evaluating functional hearing losses. Furthermore, few objective tests have been developed for supra-threshold tests of speech, frequency, or intensity discrimination.
One form of objective audiometry uses auditory evoked potentials. Auditory evoked potential testing consists of presenting the subject with an acoustic stimulus and simultaneously or concurrently sensing (i.e. recording) potentials from the subject. The sensed potentials are the subject's electroencephalogram (EEG) which contain the subject's response to the stimulus if the subject's auditory system has processed the stimulus. These potentials are analyzed to determine whether they contain a response to the acoustic stimulus or not. Auditory evoked potentials have been used to determine auditory thresholds and hearing at specific frequencies.
One particular class of auditory evoked potentials is steady-state evoked potentials (SSAEPs). The stimulus for the SSAEP consists of a carrier signal, which is usually a sinusoid, that is amplitude modulated by a modulation signal which is also usually a sinusoid. The SSAEP stimulus is presented to the subject while simultaneously recording the subject's EEG. If the auditory system of the subject responded to the SSAEP stimulus, then a corresponding steady-state sinusoidal signal should exist in the recorded EEG. The signal should have a frequency that is the same as the frequency of the modulation signal (i.e. modulation frequency). The presence of such a corresponding signal in the EEG is indicative of a response to the SSAEP stimulus. Alternatively, the phase of the carrier signal may be frequency modulated instead of or in addition to amplitude modulation to create the SSAEP stimulus.
The SSAEP stimulus is sufficiently frequency-specific to allow a particular part of the auditory system to be tested. Furthermore, the SSAEP stimulus is less liable to be affected by distortion in free-field speakers or hearing aids. Typical modulation frequencies which are used in SSAEP stimuli are between 30 to 50 Hz or 75 to 110 Hz. The latter range may be particularly useful for audiometry because at these rates, the SSAEP responses are not significantly affected by sleep and can be reliably recorded in infants. Furthermore, SSAEP responses at these rates result in audiometric threshold estimates that are well correlated with behavioral thresholds to pure tone stimuli. In SSAEP testing, the presence or absence of an SSAEP response to an SSAEP stimulus can be determined using several statistical techniques.
However, objective audiometry employing SSAEP testing is time-consuming because the amplitude of the SSAEP response is quite small compared to the background noise which is the subject's ongoing brain activity (i.e. EEG) while the test is being conducted. The SSAEP response thus has a small signal-to-noise ratio (SNR) which makes it difficult to detect the SSAEP response in a short time period. One technique to reduce SSAEP testing time is to use a multiple SSAEP stimulus which combines several SSAEP test signals (i.e. where a test signal is meant to mean one SSAEP stimulus). The potentials sensed from the subject during the presentation of the multiple SSAEP stimulus contains a linear superposition of SSAEP responses to each SSAEP test signal in the multiple SSAEP stimulus. This makes it possible to record the SSAEP responses to multiple (e.g., four or eight) stimuli in the same time that it takes to record the response to a single stimulus. Therefore, this technique results in a reduction of test time since the SSAEP responses to several SSAEP test signals may be detected concurrently. However, the SNR for each SSAEP response is still small and the testing time for recording the response to a single SSAEP stimulus has not been reduced. To reduce the SSAEP test time techniques are required to either increase the amplitude of the SSAEP response and/or decrease the amplitude of the noise that is recorded along with the SSAEP response. A more sensitive statistical method that can detect SSAEP responses with small SNRs would also be useful.
While objective testing identifies that a subject has a hearing loss, the next step is usually to treat the subject by providing them with a hearing aid. However, if the subject is an infant, a method is required to objectively adjust the hearing aid since this cannot be done with conventional subjective methods. Some objective methods have been developed such as determining the real-ear insertion gain when a hearing aid is in place. However, this method is only useful if one knows the actual unaided audiometric thresholds of the subject so that the hearing aid can be adjusted to match prescriptive targets. Furthermore, placement of a probe-tube in an infant can be challenging. There have also been methods based on click evoked auditory evoked potentials (i.e. wave V of the click-evoked ABR) but the stimuli used in these methods are restricted to certain frequency ranges and do not test the ability of the hearing aid to process continuous signals like speech. Accordingly, there still remains a need for an objective method to measure the benefits of a hearing aid in patients where behavioral thresholds and real-ear measurements are difficult to obtain.